Some components of refrigeration compressors are supported by bearings. To achieve reliable operation for long periods of time, bearings require lubrication by a lubricant with adequate viscosity. In a refrigeration system, this is provided by the use of a suitable oil. Oils typically used in refrigeration systems form solutions with refrigerants. During long periods of non-operation, refrigerant will tend to move into solution with the oil, thereby severely diluting residual oil in the bearings and any other components requiring lubrication as well as oil stored in the oil reservoir. Long periods of compressor non-operation can also cause oil to be drained from the bearings altogether, resulting in no bearing lubrication at all upon start up.
Two methods are known for countering dilution of oil in the oil reservoir during periods of non-operation. In one method, all flow lines connected to the reservoir are closed via automatic control of solenoid-actuated valves. In this manner, substantial migration of refrigerant from the system to the oil reservoir, and subsequently into solution with the oil, is prevented. The other method uses an electric heater to heat the oil in the oil reservoir, thereby raising its temperature. It is the nature of the oil-refrigerant solubility relationship that increasing temperature causes decreasing refrigerant concentration in the oil-refrigerant solution.
By use of at least one of these methods, oil of adequate viscosity can be maintained in the reservoir. However, after long periods of non-operation, oil can be completely washed out from the bearings or, if some residual oil still remains in the bearings and other components, it will still be severely diluted by refrigerant migration. If the compressor is started after such a period, the bearings or other components will operate for some period of time with no lubricant, or lubricant of insufficient viscosity, causing metal-to-metal contact between parts. This can result in wear, ultimately shortening the useful life of the compressor. Additionally, in some compressor refrigeration systems, pressure differences may be used to develop lubrication flows. In such systems some time may be required after start up to develop pressure differences adequate for establishing lubrication flows. During this time, residual oil in the bearings and other components may be depleted if the amount is too little, thereby resulting in wear.
Such dilution by refrigerant of residual oil in bearings and other components or depletion of residual oil before or shortly after start up can be countered by lubricating the bearings and other components prior to start of operation with oil of adequate viscosity supplied from the oil reservoir, the adequate viscosity having been developed by any means including the two methods described above. One method of accomplishing pre-start lubrication is by use of a positive displacement pump (with suitable piping) which is activated prior to start up, thereby drawing lubricant from the oil reservoir and delivering it to the bearings and other components. A positive displacement pump suitable for this purpose adds its own reliability risk as well as substantial cost.
Prior to shut-down, pressurized oil, or oil-rich oil-refrigerant solution, possibly with some refrigerant gas, is isolated from the rest of the refrigeration system. The isolated oil, being at pressures developed during operation, is at a pressure that is higher than the pressure existing in the bearing cavities and other components at the time of start up and is maintained at this higher pressure throughout the period of shut-down via application of heat using heaters. Maintenance of the pressure level may not be exact. Some further elevation of pressure may occur or some decrease in pressure may occur if the initial pressure level exceeds that which is required. Preliminary to restarting the refrigeration system, the state of isolation of this oil is ended by placing the oil in fluid communication with bearings and possibly other components to be lubricated.
Flow of oil results by virtue of its pressure being higher than the pressure at the bearings and other components, thereby accomplishing pre-start lubrication.
It is an object of this invention to provide lubrication prior to start up without the use of a positive displacement pump.
It is another object of this invention to provide a refrigeration screw compressor system with high reliability and long life by avoiding failure of a positive displacement pre-start lubrication pump and by eliminating component failure modes due to start up with lubrication of insufficient viscosity or due to running with an insufficient amount of lubrication during a period of time just after start up but before adequate lubrication flow is established.
It is a further object of this invention to provide a method and apparatus for lubrication delivery prior to start up that is compatible with the normal operation of the lubrication system. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, at or just prior to shut-down, or at least prior to a significant pressure equalization in a refrigeration system, a vessel containing pressurized oil or a pressurized oil-rich oil-refrigerant solution, possibly in combination with some refrigerant gas, is isolated from the rest of the refrigeration system and is then maintained in a pressurized state while the refrigeration system is not operating. Preliminary to start up of the refrigeration system, the pressurized oil and refrigerant gas or oil-rich oil-refrigerant solution, is placed in fluid communication with the bearings and any other components requiring pre-start lubrication. Pre-start lubrication then results as oil flows to the bearings and possibly other components due to the pressure difference between the vessel containing oil and the regions to be lubricated.